The present application relates generally to systems and methods for seismic exploration, including the acquisition and/or processing of seismic data to estimate properties of the Earth's subsurface.
The principal type of data used to explore for oil and gas resources is seismic reflection data that image subsurface geology. There are three seismic wave modes that can be used for subsurface imaging—a compressional-wave (P) mode and two shear-wave modes (SV and SH). When geophysicists acquire seismic data that have all three of these modes, the data are called full elastic-wavefield data. Full elastic-wavefield data are acquired by deploying three separate orthogonal seismic sources at every source station across a prospect area. One source applies a vertical force vector to the Earth, a second source applies a horizontal force vector in the inline (X) direction, and a third source applies a second horizontal force vector in the crossline (Y) direction.
The wavefields produced by each of these three orthogonal-force sources are recorded by 3-component geophones that have orthogonal (XYZ) sensing elements. The resulting data are called 9-component data because they consist of 3-component data produced by three different sources that occupy the same source station in sequence, not simultaneously. Full descriptions and illustrations of the sources, sensors, and field procedures used to acquire full elastic-wavefield data can be found in Chapter 2, Multicomponent Seismic Technology, Geophysical References Series No. 18, Society of Exploration Geophysicists, authored by B. A. Hardage, M. V. DeAngelo, P. E. Murray, and D. Sava (2011). Vertical, single-component, surface-based geophones are used for the purpose of acquiring P-wave seismic data.
Marine seismic data are generated by an air gun source (e.g., an air gun array) towed a few meters (e.g., 3 to 15 m) below the sea surface. Data are recorded by a long cable (e.g., as long as 10 or 15 km) that has hydrophones spaced at intervals of a few meters (e.g., 10 to 20 m). Several of these hydrophone cables can be towed by the same boat that tows the air guns, or the source and the hydrophone cables can be towed by separate boats. Sometimes there are two cable boats moving along parallel tracks, maybe 6 or 8 km apart, and each towing 10 or more cables as long as 15 km that span a lateral distance of 1 to 2 km. In these modern long-offset, multi-azimuth marine surveys, there are 2 to 4 source boats stationed around the cable boats. The whole procedure involves a small armada moving at a slow speed with each boat performing its assignment with precise GPS positioning and atomic-clock timing. The amount of data recorded across a large survey area can be staggering.
Water has a shear modulus of zero, thus S waves cannot propagate in sea water. Because a marine source and receiver are in a water layer, marine seismic data are considered to be only P-wave data.